The description relates to dual-polarity mass spectrometers
Mass spectrometers can be used to determine the identities and quantities of components that make up a solid, gas, or liquid sample. A mass spectrometer may use the mass (m) to charge (z) ratios of ions to separate and analyze the ions. In one example, a time-of-flight mass spectrometer includes an acceleration region having electrodes that generate an electric field for accelerating either the positive ions (cations) or negative ions (anions) and direct them toward one end of a flight tube. Heavier ions travel at a slower speed while smaller ions travel at a higher speed in the flight tube. The ions are detected by a sensor at the other end of the flight tube. The m/z ratios can be derived based on the amount of time that it takes for the ions to travel the length of the flight tube.
In general, both positively and negatively charged particles are produced from a sample during an ionization process. Single-polarity mass spectrometers can be configured to measure either positive or negative ions, but not both, at a given time. Such measurements may not be able to capture all of the information of the sample, and may lose some information on the types and quantities of ions. Dual-polarity mass spectrometers can measure both positive and negative ions at the same time. An example of a dual-polarity mass spectrometer is an aerosol time-of-flight mass spectrometer that determines the size and chemical composition of aerosol particles by accelerating the particles through a nozzle and skimmers to produce a well-defined beam of particles. The particles are maintained electrically neutral until they reach an ionization location, upon which the neutral particles are irradiated by a laser and produce positively and negatively charged small molecules. The charged molecules are analyzed by a bipolar, time-of-flight mass spectrometer having two flight tubes, each for analyzing the positive and negative ions, respectively.